JTrade names or manufacturers' names are used in this report for identification only. This usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. SummaryThe purpose of the present work was threefold: (1) to determine if a free-stream turbulence length scale existed that would cause the greatest augmentation in stagnation-region heat transfer over laminar levels; (2) to investigate the effect of velocity gradient on stagnation-region heat transfer augmentation by free-stream turbulence; and (3) to develop a prediction tool for stagnation heat transfer in the presence of free-stream turbulence. Heat transfer was measured in the stagnation region of four models with elliptical leading edges that had ratios of major to minor axes of 1: 1, 1.5: 1, 2.25: 1, and 3: 1. Five turbulence-generating grids were fabricated; four were square mesh, biplane grids made from square bars. The fifth grid was an array of fine parallel wires that were perpendicular to the model spanwise direction .Heat transfer data were taken at Reynolds numbers ranging from 37 000 to 228 000. Turbulence intensities were in the range of 1.1 to 15.9 percent while the ratio of integral length scale to leading-edge diameter ranged from 0.05 to 0.30. Stagnation-point velocity gradient was varied by nearly 50 percent.Stagnation-region heat transfer augmentation was found to increase with decreasing length scale but no optimum length scale was found. Heat transfer augmentation due to turbulence was found to be unaffected by the velocity gradient near the leading edge. A correlation was developed that fit heat transfer data for the square-bar grids to within ±4 percent.
Local convective heat transfer coefficients were measured from a smooth NACA 0012 airfoil having a chord length of 0.533 m (21 in.). The measurements were made both in flight on the NASA Lewis Twin Otter icing research aircraft and in the NASA Lewis icing research tunnel (IRT). Flight data were taken for the smooth airfoil at Reynolds numbers based on chord in the range 1.24 X 10 6 to 2.50 x 10 6 and at various angles of attack up to 4 deg. During these flight tests, the freestream velocity turbulence intensity was found to be very low (< 0.1%). Wind tunnel data were acquired in the Reynolds number range 1.20 x 10 6 to 4.52 x 10 6 and at angles of attack from -4 to +8 deg. The turbulence intensity in the IRT was 0.5-0.7% with the cloud-generating sprays off. A direct comparison between the results obtained in flight and in the IRT showed that the higher level of turbulence intensity in the IRT had little effect on the heat transfer for the lower Reynolds numbers but caused a moderate increase in heat transfer at the higher Reynolds numbers. Turning on the cloud-generating spray nozzle atomizing air in the IRT did not alter the heat transfer. The present data were compared with leading-edge cylinder and flat plate heat transfer correlations that are often used to estimate airfoil heat transfer in computer codes.
Heat transfer measurements have been made in the stagnation region of a flat plate with an elliptical leading edge. The radius of curvature at the stagnation point was similar to that of a first stage turbine vane airfoil used in a large commercial high-bypass turbofan engine. The airfoil was mounted downstream of an arc segment of a dual-annular combustor similar to the type used in an advanced turbine engine. Testing was done in air at atmospheric temperature and at pressures up to 376 kPa to simulate the vane leading edge Reynolds number seen in the engine. Spanwise average stagnation region heat transfer was measured with an electrically heated aluminum strip. Turbulence intensity, length scale and isotropy were measured using standard 2-wire hot wire probes. The combustor contained two annular rows of fuel-air swirlers which were aligned in the radial direction. Both heat transfer and hot wire data were taken at two circumferential positions; one directly downstream of a pair of swirlers and one half way between two pairs of swirlers. Reynolds number based on vane leading edge diameter was varied from 51000 to 160000. The maximum Reynolds number for turbulence measurements was limited to 87000. Turbulence intensity averaged over all test conditions was found to be 31.6%. Average axial, integral length scale was 1.29 cm, which gave a length scale-to-leading edge diameter ratio of 1.08. The turbulence was found to be nearly isotropic with the average ratio of axial to circumferential fluctuating components of 1.15. Heat transfer augmentation above laminar levels was found to vary from 34 to almost 59% depending on the Reynolds number. No effect of circumferential position was found. The heat transfer augmentation was found to be well predicted by a correlation derived from grid generated turbulence.
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